Electroluminescent lamp



July 10, 1962 E. L. MAGER 3,043,978

ELECTROLUMINESCENT LAMP Original Filed June 9, 1951 INVENTOR ERIC L.MAGER 32mm M ATTORNEY United States Patent 3,043,978 ELECTROLUMINESCENTLAMP Eric L. Mager, Beverly, Mass., assignor to Sylvania ElectncProducts Inc., Salem, Mass., a corporation of Massachusetts Continuationof application Ser. No. 230,738, June 9, 1951. This application Oct. 24,1958, Ser. No. 769,511

2 Claims. (Cl. 313-408) The invention relates to electroluminescentlamps and to the manufacture thereof.

Such lamps have a light-transmitting piece of material with alight-transmitting, electrically-conducting surface, a mixture ofphosphor and dielectric substance coated thereover, and a conductivebacking layer over the phosphor. Light is emitted from the device when avoltage is applied between the conductive backing layer and theconducting surface of the material.

I have found that the deterioration in light out-put of such lampsduring life can be greatly reduced by the use.

of a thin dielectric coating over the phosphor-dielectric layer, that isbetween the latter layer and the conductive backing coating. Thedielectric coating can be of lacquer, wax, plastic or other material ofhigh resistivity, dielectric constant, and dielectric strength, andinactive chemically with the backing layer and the phosphor-dielectriclayer. Such a coating would be expected to cause a diminution in lightoutput from the lamp, because of the voltage drop in the coating, but Ifind that instead the light output is about the same as that without thecoating, at least after a short aging period.

The maintenance of light output during life can be still furtherincreased by the use of an alloy of aluminum and copper as the materialof the conductive backing layer.

One specific embodiment of the invention is shown partly in section andpartly cut away in the accompanying FIGURE.

The glass plate 1 in that embodiment carries on its surface thelight-transmitting electrically-conductive layer or coating 2, incontact with the metal strip 3 which preferably extends over thecorresponding edge of the glass plate 1, in the L-shape shown. A similarL-shaped metal strip 4 is aifixed to the opposite edge of the glassplate -1 as shown, it being affixed to a portion thereof to which theconductive coating 2 does not extend. There is a nonconducting spacebetween the edge of layer 2 and metal strip 4. At least part of thisnon-conducting space is covered by the phosphor-dielectric layer 5,which extends as a coating over the conductive layer 2, or by thedielectric coating 6 on the phosphor layer 5. The conductive backinglayer 7 is applied over the dielectric layer 6, at least in part, andextends into contact with the metal strip 4, although it must not alsoextend into contact with the metal strip 3 which is connectedelectrically to the transparent conductive layer 2, as that would simplyshort circuit the device.

The conducting surface or layer 2 should be durable, inactive chemicallywith respect to the other materials used in the lamp, and of goodlight-transmission. Such a durable, chemically-inactive conductingsurface 2 of good light-transmission, either transparency ortranslucency, is difiicult to obtain, because good electric condnctorsare generally good reflectors of light, rather than transmitters of it.However, although other coatings may be used, I find that a particularlyeffective conductive surface can be provided by heating the glass andexposing it while hot to vapors of the chlorides of silicon, tin, ortitanium, and afterward placing the treated glass in a slightly reducingatmosphere. Where the application in the vapor state is not convenient,good results may be obtained by mixing stannic chloride with absolutealcohol 3,643,978 Patented July 10, 1962 and glacial acetic acid anddipping the heated glass plate into the boiling mixture.

The resultant conductive surface appears to be a layer 2 of stannic (orsilicic or titanic) oxide, probably to some extent at least reduced to aform lower than the dioxide, although the exact composition is diificultto determine.

The resistance of the conductive surf-ace 2 should be low enough toavoid excessive power loss therein at the current used. A surface havinga resistance of several hundred ohms per square, that is a resistance ofseveral hundred ohms taken between the entire opposite edges of a squareof the material, has been found satisfactory, although the permissibleresistance can be varied Widely.

One side of the glass plate 1 can be completely coated with theconducting layer 2, if desired, and then the portion of layer 2 in thevicinity of the position desired for the application of the metal strip4, which must be out of electrical contact with said conducting layer 2,can be removed by applying to that portion a mixture of zinc dust anddilute acid, which can be applied with a spatula. The acid used can beone which is capable of emitting nascent hydrogen by reaction with thezinc dust. A solution of three parts Water and one part sulfuric acid byvolume, mixed with sufiicient zinc dust to form a thick slurry has beenfound very effective.

Other methods can be used for removing a part of the conductive layer orthe original application of the layer can be confined to the desiredportions by shielding or other means, if desired.

The metal strips 3 and '4 are coatings painted onto the glass piece 1(over the conducting layer 2 in the case of strip 3) by using one of theconducting silver paints of the firing type known in the art andcommonly on the market. After application, the paint is fired to theglass at about 1000 F.

Silver paints of the air-dried type can be used, but do not afford thepermanent metal-to-glass bond that can be obtained with the firedpaints. Spraying and other forms of metal deposition can be used insteadof painting, if desired. A copper coating can be plated over the firedsilver, for example from one of the usual acid plating baths, to providea still more desirable surface and one to which electrical connectingwires can easily be soldered.

if desired both contact strips 3 and 4 may be placed along differentportions of a single edge of the glass plate 1, the conductive layer 2being in contact with the first of said strips and out of contact withthe other, and conversely, the backing layer 6 being out of contact withthe first of said contacts but in contact with the second. The contactstrips 3 and 4 need not extend all the way over the edge to the oppositeface 8 unless desired. 7

The phosphor, in the form of fine particles, is suspended in a suitabledielectric medium to form the layer 5 of phosphor particles embedded indielectric. For example, in one embodiment we have dissolved about 3 /2grams of quarter-second nitrocellulose as a dielectric material insufficient butyl acetate to make 17 /2 cc. of the resulting solution,with about 6 /2 grams of castor oil to act as the plasticizing componentof plasticizer and as the dielectric, and about 9 cc. each of toluol,ethyl acetate and butyl acetate added to thin the solution, in whichabout 13 grams of phosphor are suspended. The suspension is sprayed orotherwise applied to the conductive glass layer 2, to a thickness of afew thousandthsof an inch.

The dielectric layer 6 can be applied in a similar manner over thephosphor-dielectric layer 5, that is by spraying, painting or the like.In the specific example described, the layer 6 was of clear lacquerapplied by spraying to a thickness of about 15% that of thephosphordielectric layer, although other thicknesses can be used withgood results. The layer 6 should, however, be as thin as possible"consistent with a good maintenance, as an extremely large thicknessmight reduce the luminescent brightness of the lamp for a given voltage.

The lacquer used in the specific embodiment being described vvas made bymixing wet quarter-second viscosity nitrocellulose equivalent to about50 grams of dry nitrocellulose, there being about 25 grams of ethanolpresent to wet the material, with enough butyl acetate to make up about250 cc. of solution. 50 grams of castoroil was added, and the resultantliquid was then diluted with about 400 cc. of a solvent of equal partsof butyl acetate, ethyl acetate, and toluol.

Castor oil isavailable in a wide variety of acid numbers from about 20dowmbut the best light maintenance from the. lamp will be obtained withmaterial of'thc very low acid numbers, less than 3 or 4.

The backing layer 7 is of a good conducting material, such as metal, andpreferably of a good reflecting metal such as aluminum, which will notreact appreciably with the phosphor or embedding material used. Thebacking layer 6 is preferably of low resistance and can be applied inany convenient manner, care being taken not to damagethephosphor-dielectric layer. Best results have been obtained byvacuum-deposition of the metal. The glass plate :1,'with its conductivesurface 2 coated with the embedded phosphor layer 5, is'placed in a belljar and coated over the phosphor layer with an evaporated metal after asuitable vacuum has been obtained. The backing layer 7 can also bedeposited effectively by spraymg.

In the particular example described, the conductive backing layer 7was'an alloy of 87% aluminum and 13 copper by weight. The addition ofcopper to the aluminum improves the lumen maintenance of the lamp, thatis, improves the maintenance of its light output during life. Theaddition of the copper also has the effect of greatly toughening thealuminum film and increasing its durability. Other proportions of copperand aluminum can be used than those given, but the amount of copperadded should not be great enough to seriously reduce the reflectivity ofthe aluminum.

A transparent conductive plastic, or a transparent plastic coated with atransparent conductive layer, can be used as the piece 1, if desired.Where the piece 1 is conembedding dielectric material, for example, awax or plastic of reasonably high dielectric constant, dielectricstrength and resistivity.

The thickness of the various layers can be altered to "suit variousvoltageconditions and the like. The voltage will depend on the phosphorused, the thickness and composition of the layers 5 and 6, and thebrightness desired, but voltages between 25 volts and 2500 volts andeven higher have been used. A lamp operable from a 1l0- volt alternatingcurrent power line can be made with the conducting surface 2 of athickness of about a wavelength of light, with a phosphor layer 5 ofabout two one thou-,

sand-tbs of an inch, and a metal layer 4 of a fraction of a thousandthof an inch. The plate 1 can have any convenient thickness and must belight transmissive.

The phosphor used must be one capable of electroluminescence, that is ofemitting light or other desired radiation on excitation by a varyingelectric field when the phosphor is in a dielectric medium. Suitablephosphors are described, for example, in applications filed concurrentlyherewith by Keith H. Butler and Horace H. Homer as joint inventors, andby Keith H. Butler as a sole inventor, although other electroluminescentphosphors can be used.

The portion 9 of the metal backing layer 7 is preferably reinforcedwhere it passes over the edge of the phosphordielectric layer 5, or thedielectric layer 6, to prevent breakage of the layer along that edge.Painting a thin stripe of one of the well-known air-drying conductivesilver paints over the backing layer 7 along the portion that is inregister with said edge is a satisfactory strengthening means for thatportion of the lamp.

The entire back of the unit is preferably coated with an insulatingprotective layer 8, for example of lacquer.

This application is a continuation of application Serial No. 230,738,filed June 9, 1951, now abandoned.

What I claim is:

1. An electroluminescent lamp comprising a piece of light-transmittingmaterial, a light-transmitting electrically conductive surfiace on saidpiece, a layer of an electroluminescent phosphor in a dielectric mediumon said conductive surface, and a conductive backing layer of an alloyof aluminum and copper on said layer.

2. The combination of claim 1, in which the alloy contains 87 aluminumand 13% copper.

New Phenomenon of Electroluminescence and its Possibilities forInvestigation of Crystal Lattice, Prof. G. Destriau, PhilosophicalMagazine, October 1947, vol. 38,

1. AN ELECTROLUMINESCENT LAMP COMPRISING A PIECE OF LIGHT-TRANSMITTINGMATERIAL, A LIGHT-TRANSMITTING ELECTRICALLY CONDUCTIVE SURFACE ON SAIDPIECE, A LAYER OF AN ELECTROLUMINESCENT PHOSPHOR IN A DIELECTRIC MEDIUMON SAID CON-